I'm doing table top macro in a home made light tent using 100W equivalent CFLs in articulated arm lamps clamped to a plywood sheet on which the tent sits.

The lights are right against the sides/top of the diffuser material (thin white fabric hand towels).

I don't believe I'm getting enough light. I'm shooting [at a defunct gnat] at f8 in aperture priority at ISO 100. The lens is a Tokina 100mm macro on a full set of ProMaster extension tubes. I'm getting .50 to .60 sec exposures.

There's a certain amount of flex to the floor, and I can see camera movement, not from the tripod, but from the floor itself.

I'm using DSLR Controller on an ASUS tablet to do focus stacking. I start the stack and [try] to walk away as gently as I can. I of course have no control over ambient noise/vibration in the apartment. I can see certain frames seriously out of focus, due apparently to movement.

Are my bulbs not powerful enough; is my ISO too low? Diffuser material taking too much light? All three?
It seems that my shutter speed is too slow for the conditions and I'm looking for a way to ameliorate that without going to flash.

The lights are right against the sides/top of the diffuser material. . . . .

Don’t do this! If your lights are right up against the diffuser, you’re not letting the diffuser do its job. The idea is to scatter light evenly across as much of the diffusion material as possible, so that the lit portion of the diffuser, from the viewpoint of the subject, will be very large. It's this lit portion of the diffusion material that the subject "sees" as the light source. Of course, moving the lights back will make the problem you asked about—insufficient illumination—worse.

So instead of moving the lights farther away from the diffuser, is it possible in your arrangement to move the diffuser closer to the subject? I don't know how big your light tent is, but the one I use for product photography is two feet or more square. This is great for object about the size of my hand, but way too big for a gnat--even a big gnat. If your light tent is like mine, try something much smaller, such as a ping-pong ball or a small cylinder of diffusion material an inch or two diameter. This will let you position your lights far enough from the diffusion material to light most of it, but close enough to your subject to concentrate the illumination.

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. . . diffuser material (thin white fabric hand towels).

I have no idea how opaque these towels are, but suspect their level of light loss is higher than ideal. In my experience, good diffusion material costs about two stops of light; if your thin white fabric kills more light than that, you probably want to consider something else.

For a small cylinder, a double layer of tracing paper might work well (this has been recommended in our fora, but i have not tried it). A thread discussing diffusion materials is here.

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I'm shooting [at a defunct gnat] at f8 in aperture priority at ISO 100. The lens is a Tokina 100mm macro on a full set of ProMaster extension tubes. I'm getting .50 to .60 sec exposures.

If you mean "nominal" f/8: ("Nominal," because with the extension tubes added and lens helicoid likely extended, your effective aperture is surely less—probably f/16 or smaller), you are well into diffusion territory, which costs resolution, and also costs light due to stopping down. In this case, I’d suggest opening up your aperture to 1½ stops down from wide open, to gain light and sharpness. This will force you to use a smaller stepping increment, and produce more images to stack, but since your setup is automated, that’s likely not a problem.

On the other hand, if you mean "effective f/8": After considering bellows factors are considered, you are likely shooting at optimum aperture or close to it already, so no problem.)

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There's a certain amount of flex to the floor, and I can see camera movement, not from the tripod, but from the floor itself.

When you are photographing in the realm of gnat-sized objects, you're working at magnifications where the traditional approach of a tripod and mechanically-separate subject is often frustrating. It’s much more fault tolerant if you mechanically couple the camera and subject on a single platform, such as a piece of wood, stone, or metal. With a coupling platform, any outside force that jiggles the camera also jiggles the subject in unison, reducing or eliminating apparent camera motion. This is why so many of us have built macro rigs; these rigs vary a lot in detail, but all the good ones have a solid platform linking the camera and subject.

If that doesn’t solve your movement problem, the next thing to do is float that platform on something squishy-and-not-bouncy, such as Sorbothane brand elastomer feet, properly chosen to match the mass of your rig. Ideally, this lets the world jiggle however it may, while your macro rig floats placidly on top, isolated from the vibrations of the world. In such a case, you may be able to use shutter speeds that are much longer than the approximately ½ second you’re currently using.

BTW, in this shutter speed range, vibration induced by mirror movement and shutter slap can produce considerable blur. Hopefully, you’ve avoided these by using mirror-up or exposure delay, and some form of electronic first curtain shutter.

So instead of moving the lights farther away from the diffuser, is it possible in your arrangement to move the diffuser closer to the subject? I don't know how big your light tent is, but the one I use for product photography is two feet or more square. This is great for object about the size of my hand, but way too big for a gnat--even a big gnat. If your light tent is like mine, try something much smaller, such as a ping-pong ball or a small cylinder of diffusion material an inch or two diameter. This will let you position your lights far enough from the diffusion material to light most of it, but close enough to your subject to concentrate the illumination.

I fortunately built my light tent from PVC, and it can easily be reconfigured for size and shape.

Chris S. wrote:

I have no idea how opaque these towels are, but suspect their level of light loss is higher than ideal. In my experience, good diffusion material costs about two stops of light; if your thin white fabric kills more light than that, you probably want to consider something else.

They transmit quite a bit of light. The tent and diffuser materials were just fine for what I was photographing previously. There was no problem with things like handguns and things of a similar size. It wasn't until I started photographing ~1/8" gnats that illumination became a problem.

I have switched to thin packing foam sheets from OfficeMax. They were quite good for flash diffusing. They transmit significantly more light than the hand towels.

Chris S. wrote:

If you mean "nominal" f/8: ("Nominal," because with the extension tubes added and lens helicoid likely extended, your effective aperture is surely less—probably f/16 or smaller), you are well into diffusion territory, which costs resolution, and also costs light due to stopping down. In this case, I’d suggest opening up your aperture to 1½ stops down from wide open, to gain light and sharpness. This will force you to use a smaller stepping increment, and produce more images to stack, but since your setup is automated, that’s likely not a problem.

Yes, it's "nominal" f/8. I'll give the larger aperture a try.

Chris S. wrote:

When you are photographing in the realm of gnat-sized objects, you're working at magnifications where the traditional approach of a tripod and mechanically-separate subject is often frustrating. It’s much more fault tolerant if you mechanically couple the camera and subject on a single platform, such as a piece of wood, stone, or metal. With a coupling platform, any outside force that jiggles the camera also jiggles the subject in unison, reducing or eliminating apparent camera motion. This is why so many of us have built macro rigs; these rigs vary a lot in detail, but all the good ones have a solid platform linking the camera and subject.

I'm considering building a photography specific table using thick plywood or butcher block and 3/4" plumbing pipe and floor flanges. This would be both more stable and hopefully give me enough room to set the tripod (with leg sections collapsed) on the table itself. This will however have to wait until I can afford the expense.

Chris S. wrote:

BTW, in this shutter speed range, vibration induced by mirror movement and shutter slap can produce considerable blur. Hopefully, you’ve avoided these by using mirror-up or exposure delay, and some form of electronic first curtain shutter.

The last time I looked, I had the camera set for mirror lockup. I'll have to check to be sure.

You'd be better off resting your butcher block on a semi inflated inner tube. That way vibrations from the floor don't get transmitted into your rig.. Only very low frequencies make it through, which won't bother you if everything on the boat moves together.

You have a Canon Txi iirc? Use Live View, which uses Electronic Front Shutter Curtain, It'll cycle the mirror after the shot though, so allow say 4 seconds between shots. Mirror lock doesn't help hugely if you do that, it's the front shutter curtain you have to e careful with.
Either that or
NOT using Live view,
Do use Mirror lock, wait a few seconds after the mirror locks, then use quite a long exposure - several seconds. Then any shutter-induced blur is a tiny fraction of the exposure so you don't see it.

Snag with Live View for extended periods is you can get "hot" pixels._________________Chris R

You'd be better off resting your butcher block on a semi inflated inner tube. That way vibrations from the floor don't get transmitted into your rig.. Only very low frequencies make it through, which won't bother you if everything on the boat moves together.

I'm working in my living room, so that wouldn't work out for me.

A solid table with enough room for the tripod (or a mount clamped to the table should be adequate, at least for this application. If I was really concerned about vibration as opposed to outright movement, I could add some sort of rubber dampening material between the legs and the table top.

ChrisR wrote:

You have a Canon Txi iirc? Use Live View, which uses Electronic Front Shutter Curtain, It'll cycle the mirror after the shot though, so allow say 4 seconds between shots. Mirror lock doesn't help hugely if you do that, it's the front shutter curtain you have to e careful with.

I'm doing automated focus stacking with DSLR Controller and an Android tablet. I get live view on the tablet.

sounds like a bad idea. The floor vibrates all the time - at least mine does.

I had a long answer, which the forums ate when they crashed a while ago.

To make a long story short, I intend to build a solidly constructed table to hold BOTH the subject AND the tripod. My current setup is not amenable to that, there being both inadequate room for the tripod and excessive distance between the camera and the subject.

I had a long answer, which the forums ate when they crashed a while ago.

Sorry about that. I definitely sympathize, since I ended up wasting about an hour trying to solve what turned out to be somebody else's problem. The root cause was something messed up at our service provider that affected multiple servers. My commercial site went dead too. These outages are always annoying, but when I do the calculation I'm reminded that 99.9% availability still means 8 hours per year down.

Sorry about that. I definitely sympathize, since I ended up wasting about an hour trying to solve what turned out to be somebody else's problem. The root cause was something messed up at our service provider that affected multiple servers. My commercial site went dead too. These outages are always annoying, but when I do the calculation I'm reminded that 99.9% availability still means 8 hours per year down.

--Rik

I spend all day doing, among other things, trying to fix downed servers.